You can teach yourself piano by ear

Developed by Robin Jarman to enable anyone to accompany songs on the piano without music, change key when a song is pitched too low or too high, and provide accompaniment when only the melody and chord symbols are given. It is ideal for teachers, students, parents, and members of the community who would like to accompany sing-alongs. The book does assume that the student can sing in tune, can read simple rhythm and notation, is familiar with the treble and bass clefs and the piano keyboard, and has a sense of humor and a desire to learn. The audio and video downloads available online walk you through the exercises, providing additional instruction and help.